The electrophotographic method is a technique widely used in image forming apparatus such as an electrophotographic facsimile and an electrophotographic printer. As the electrophotographic method, a system using a photoconductive insulator is generally used. In this system, a toner image is once formed on the photoconductive insulator by using a toner, a resin powder colored with a pigment or a dye, and transferred onto a recording medium such as paper. Then, the toner is fused on the recording medium by heat, pressure or light, followed by solidification to finally obtain a toner image fixed onto the recording medium. This fixing system is roughly divided into a heat roll fixing system and a flash fixing system.
In recent years, the market for on-demand printing has rapidly expanded. In the on-demand printing, it is necessary to prepare hundreds to thousands of printed materials for a sort delivery period according to the diversifying needs of customers. For such a purpose, the electrophotographic printer capable of conducting high-speed printing is suitable.
When the fixing system is studied from the viewpoint of speeding up of printing speed in order to satisfy such needs, the heat roll fixing system has a limitation on the speeding up, because a recording medium, for example, paper, on which a toner image is formed, is pressed with a heating roll to cause the liability of the paper to crinkle, resulting in the occurrence of paper clogging. Further, the toner is partly transferred to the heating roll in fixing, so that it becomes necessary to periodically clean the heating roll.
On the other hand, in the flash fixing system, optical energy given by a flash light of a discharge tube such as a xenon flash lamp is converted into thermal energy, thereby fusing a toner and fixing it onto a recording medium. Accordingly, this system has the advantage that the speeding up is easy because high-speed and non-contact fixing of the toner is possible.
In the spectral distribution of the xenon lamp generally used as the discharge tube for flash fixing, the light emission intensity in a near-infrared wavelength region of 800 nm to 1,000 nm is significantly high, and the light emission intensity in a visible region of 400 nm to 800 nm is relatively low. It is therefore required that the toner subjected to flash fixing has high optical absorption in the near-infrared wavelength region.
However, coloring agents added to color toners generally have low optical absorption in the near-infrared wavelength region. Consequently, in order to improve optical absorption to improve fixability, an infrared ray absorbent is added to the toner. As the infrared ray absorbents for the toner, there are known an aminium salt, a diimonium compound, a cyanine compound, a nickel complex compound, a phthalocyanine compound, an ytterbium oxide compound, etc. (See, for example, JP-A-61-132959 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-7-191492, JP-A-10-39535, and JP-A-11-038666). Further, a technique of adhering the infrared ray absorbent to the surface of the toner has been proposed (See JP-A-2002-156779 and JP-A-06-118694).
Patent Document 1: JP-A-2002-156779
Patent Document 2: JP-A-11-038666
Patent Document 3: JP-A-06-118694
However, the infrared ray absorbent has absorption also in the wavelength region of a red color, so that it develops a green color which is a complementary color to the red color. Accordingly, when the same pigment as contained in the infrared ray absorbent-free toner of the heat roll system is used in the toner to be subjected to flash fixing, the whole color gamut is shifted by the influence of the color of the infrared ray absorbent. The as-shifted color gamut cannot provide sufficient color reproducibility, even when adjustment is carried out with an image forming apparatus such as an electrophotographic printer. For example, the use of a green infrared ray absorbent such as naphthalocyanine raises the problem that a red color is darkened, resulting in failure to obtain a bright red color. Furthermore, the red color is a color most frequently used, so that the problem is encountered that the whole color reproducibility is impaired.